Currently, the Image: namespace on all(?) projects is 'transparent' (I don't know if that's the technical term, but it's reasonably descriptive) to the Image: namespace at commons; so if the page [[commons:Image:Foo]] exists, viewing [[haw:Image:Foo]] will display the commons page, templates and all, if [[haw:Image:Foo]] does not exist. With the advent of SUL, we have the situation where users are required to create identical or near-identical userpages on multiple wikis - precisely the situation that commons was created to resolve for images. It would therefore be emminently sensible to make the User: namespace transparent in the same way. However, commons would be an awkward choice of 'underlay' - the meta:User: namespace would be much more sensible.
As this would only be useful on wikis with CentralAuth, it makes sense to implement it there. It might be a good idea to put some checks in to confirm that a global account actually exists for each username... but then again the abuse potential is low because if a local user is not part of the global account for that username, they can override the transparent display simply by creating a page of their own in its place. I don't think User talk: pages should be treated in the same way.